Theophylline is a classic, multi-action methylxanthine bronchodilator and respiratory smooth muscle relaxant. It functions primarily through two distinct cellular mechanisms: it non-selectively inhibits phosphodiesterase (PDE) enzymes (mainly PDE3 and PDE4), which slows down the degradation of cyclic adenosine monophosphate (cAMP) and leads to continuous smooth muscle relaxation in the airways; and it acts as a competitive antagonist at adenosine receptors, blocking adenosine-mediated bronchoconstriction. Uniquely at lower plasma concentrations, it also activates histone deacetylase 2 (HDAC2), which suppresses the transcription of pro-inflammatory genes to reduce underlying airway swelling. This comprehensive pharmacological profile physically widens restricted pulmonary passages, enhances diaphragm contractility to improve breathing mechanics, stimulates the central nervous system respiratory canter, and keeps the entire pulmonary system systemically stable and functional.